This invention relates to air brakes for motor vehicles, and more particularly it is concerned with an antiskid device for air brakes which enables to apply the brake to stop the vehicle in a short braking distance without causing the hazard of skidding when the brake is applied.
The slip rate which indicates the degree of skidding of a wheel rotating on the road surface while the brake is being applied thereto can be expressed as follows: ##EQU1## WHERE Vv is the speed of the motor vehicle and Vw is the speed of the wheel. When S=1, the rotational movement of the wheel is stopped by the high braking force in spite of the fact that the motor vehicle is in operation, so that the wheel is in a locked condition. This presents a hazard because the motor vehicle tends to slip sideways. When S=0, the speed of the motor vehicle is the same as that of the wheel and no braking force is transmitted from the wheel to the road surface, so that it is impossible to stop the vehicle unless air resistance or the gradient of the road is utilized. As is well known to a person skilled in the art, S=0.2 or thereabout is a slip rate which is desirable because the hazard of sideways skidding can be avoided and the vehicle can be stopped in a short braking distance.
Antiskid devices for air brakes are designed, based on the aforesaid theory, to maintain the slip rate at about 0.2 when the brake is applied. An antiskid device of the prior art comprises a modulator mounted in a pressurized air line connecting a brake valve to brake actuators, and a signal producing means which produces a pressure reducing electric signal when the braking pressure applied by the actuators has reached a pressure increase target value and turns off the pressure reducing electric signal when the braking pressure has reached a pressure decrease target value, such modulator being actuated as the pressure reducing electric signal is turned on or off to thereby decrease or increase the braking pressure applied by the actuators, whereby the speed of the wheels can be varied to maintain the slip rate at an optimum rate of 0.2 or thereabout.
Some disadvantages are associated with antiskid devices of the prior art constructed as aforesaid. For one thing, there is a time lag of the initiation of a decrease or increase in the braking pressure caused by the operation of the modulator behind the turning on or off of a pressure reducing electric signal. This has hitherto caused a rise or a fall in the braking pressure applied by the brake actuators to an unnecessarily high or low level, with the result that skidding of a moderate degree has occurred or the braking force has not been high enough.